Mechanisms of Hemispherically Symmetric Climate Variability*

Inspired by paleoclimate evidence that much past climate change has been symmetric about the equator, the causes of hemispherically symmetric variability in the recent observational record are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset and numerical models. It was found that the dominant cause of hemispherically symmetric variability is the El Niño–Southern Oscillation. During an El Niño event the Tropics warm at all longitudes and the subtropical jets in both hemispheres strengthen on their equatorward flanks. Poleward of the tropical warming there are latitude belts of marked cooling, extending from the surface to the tropopause in both hemispheres, at all longitudes and in all seasons. The midlatitude cooling is caused by changes in the eddy-driven mean meridional circulation. Changes in the transient eddy momentum fluxes during an El Niño event force upper-tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the Coriolis torque. The eddy-driven ascent causes anomalous adiabatic cooling, which is primarily balanced by anomalous diabatic heating. Using a quasigeostrophic spherical model, forced by an imposed surface eddy disturbance of chosen wavenumber and frequency, it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude–height plane of transient eddies. Changes in zonal winds, and associated changes in the meridional gradient of potential vorticity, create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward. Tropical forcing of variability in the eddy-driven mean meridional circulation is another way, in addition to Rossby wave teleconnections, whereby the Tropics can influence extratropical climate. Unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle.